DOMTokenList: Make sure we do not add the same token twice

In case we do element.classList.add('a', 'a') we need to ensure that we
do not add the same token twice.

http://dom.spec.whatwg.org/#dom-domtokenlist-add

BUG=232448
R=tkent

Review URL: https://codereview.chromium.org/25750002

git-svn-id: svn://svn.chromium.org/blink/trunk@159235 bbb929c8-8fbe-4397-9dbb-9b2b20218538